Surplus conversion handbook, including "command sets"

Citation preview

Cat. No, 122

-by

- .

Tom Kneltel, KSFLL/WBRAAI

$3

SURPLUS CONVERSION HANDBOOK Including "Command Sets" Ed/led by

Tom Kneitel, K3.F LL/WB2AAI

COWAN PUBLISHING CORP. 14 VANDERVENTER AVE.

PORT WASHINGTON, N. Y.

Jn memory of my father.

F irst Print ing Second Printing

©

December 1964 May 1967

1964 by Cowan Publishing Corp . All rights reserved. Th is book or ony parts thereof may not be re· produced in ony form or any language without permission of the publisher.

library of Congress Catalog Card Number 64 · 893 6 Printed in the United Stoles of America by ELGIN PRESS, INC., NEW YORK

The past two decades have seen such national institutions as the 11111mu, the hula hoop, and the Edsel come i111.o being and then sink into oblivion. There are 011/,y two thin.g s left from the past, Ed Sullivan aml milila1'y sm·fJlus ratlio gear. Yes, even after 20 years it is still possible to work ufJ complete (and good) ham station from this equipment-and do it. for jar less loot tha1i putting a commercial£,• man11/ac/11rc1l ham station on the air. Ami. the real/,y best part of it all is the fact that the equipm ent lends itself lo giving lazy 11011-equipme11t comtructing goof-offs some man hours behind a u1a,..m .wlilering g1111. Surplus p1·ices a1·e11!t too dif]crcnt than th ey were right .after the w ar. You ctm still buy ARC-5 and 274N t.rammitters for $.5 and receivers fo1· $7, the A RT-13 (a 100 w att /one/CW transmitter for 20, 40 and 75 meters) is aflailable for a&out. $40, and the great BC-342 receiver can still be picked up /01· less them $50. The reason why these pieces of gear are so relatively i1iexpensive is that ''OU a.re going to h11ve to a.dtl some elhow grease and solder to get them in operation. Not that they at·e defective, it's just that they ru n on mch improbable things as 400 CPS or 24 volts, 01· sometimes the frequencies cove1·ed d01lt inclttde a ham band, or the crazy thing hone ond publicoddrcn fl octro-m.cchonicol lnol other·

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) Dual Transmitter Rack FT-226 Transmitter rack shock mowits FT-Z27 Modulator unit shock mounts FT-225 T riple-Receiver rack FT-220 (Used for parts only) Antenna Relay unit BC-442 (Mete r only used) Recei\·er Dynarnotor DM-32 Two major units comprise the station proper: (I) the transmitter and receiver, mounted together in rack FT-226 and, (2) the power supply, keyer and controls, built as one unit on the BC-456 modulator chassis. Shock mounts listed above arc used for the respective units. Inter-unit wiring is by means of cable and plugs. To provide receiver accomodation in the dual t ransmitter rack, a section from the receiver rack is cut and fitted into the right-hand compartment.

izes portable ope ration? And balancing a bug on the edge of :m orange crate doesn't make for the smoothest sending on the air, either. The writer must confess to his share of just such installat ions. But recently when the need arose .for an auxiliary layout at W60WP, it was decided that the new rig wou ld be different. It would be easy to put o n the air. And a few of the operating comforts of the home rig would be incorporated. Then, to add the final touch, the transmitted signal must conform to fixed station standards-no excusing a chirpy signal on the basis of "operating portable hr, OM." A11 SCR-274N-the familiar Command SetWd~ chosen as the basic equipment, both from the stai 1dpoint of utility and availability at reasonable cost. Either 40-tors making up the assembly. The a.c. connection is made by m eans of a line cord directly to this supply rather than through the 'cceiver power socket. Since the supply is a.c.d.c. a recommended hookup method is shown in the c ircuit diagram. In this way, the receiver may be used independently of the portable set-up by merely grounding the volwne control line at tJ1c rear socket (/>i11 1 to f>i11 6). The male plug removed from the rig11t- hand section of the transmitter rack when installing the receiver compartment makes a handy shorting plug by merely soldering these two pins together. Power Supply and Keyer

The over-all schematic for this unit is shown in Fig. 3. T he mechanical and electrical layout can be seen from the photog raphs. Perhaps additional comment is needed on the power supply concerning the small filter employed. Stability of t11c converted BC-459 or BC-457 is so good that further filt ering is not required for c,w. operation. Should a BC-458 (tuned to 7 me) or an 80-meter BC-696 be used, a small input filter choke will be necessary. The lower "C" oscillators of these two u·ansmitters result in excessive ripple with the capacity filter alone. A ST4 rectifier tube is used since it fits neatly · inside the BC-456 shield. The built-in electronic keyer uses the author's "self-completing" circuit-so called since a dot or

Fig. 3. Power supply-keyer.

Cl - . 1 µf, 600 w.v. C2-.03 µf, 600 w.v. C3-.0S µf. bOO w.v. c~o µf, 150 v. filter. CS-10 1Lf. 150 v. filter. Cb- .002 µf, Rt-.5 mog . variable. R2, R3- .25 mo9 .. 1/i w.

R4--4 mo9 .. 1h w. Rs. Rb-2200 ohms. r w. R7-IO,OOO ohms, 2 w. RS-7000 ohms, I w. R9-5000 ohms, variable. R10-.25 mog .. I w. RI 1- 150 ohms. 5 w. R12--40 ohms, I w. R13-500 ohms, 1 fi w. R 14-2 me9., vorioble. R15-Rofer to text. R16- 15.000 ohms, 25 w. R17-25,000 ohms, 100 w.

Tl-Power Transformer. Ryl. Rv2-Pulm reloy, 3500 ohms. Ry3-Rofer lo lexi. Swl-A.C. switch. Sw2-H.V. switch. Sw3-Rofer to text. Sw4-K~y lover. RFC- 2112 mh. choke.

19

dash once started completes itself automatically. This assures freedom from clipping and makes for easier, more uniform sending. Electrically, the circuit consists of two stages. The first, or pulse stage, develops the basic timing volta.ge for the automatic dots or dashes. The second stage shapes the operating cycle of the keying relay to confor.m witl1 correct telegraph mark-to-space characteristics. Tubes used are SOL6s. These, toget·ber with their associated plate circuit relays, arc housed in tbe BC-456 shield enclosure. A half- wave selenium rectifier furnishe s cl.c. [or the keyer and supplies the blocJcjng bias for the transmitter oscillator keying. The .polarity of the selenium stack should be strictly observed in connecting the circuit in the manner shown. Speed and shaping controls are mounted on the ch"5sis front to the leit and right respectively of the key lever. The a.c. line switch is mounted on the speed control potentiometer. The shaping control adj us ts keying from .t ight to heavy to suit the operator's particular style. As a general rule, sending with an automatic key sounds better when adjusted on the "heavy" side since a natural tendency to exa.g gcrate letter spacing is Jess noticeable. Fixed adjustment, wiping contact relays of iden· tical characteristics are a must for the keycr circuit shown. Either short or long telephone type 2000 to 4000-ohm relays are satisfactory. High quality capacitors, preferably of the new plasticmoulded type, should be used a1 C1, C2, and Ca. R 1 in the timing circuit sets dot-to-dash ratio. Adjustment s'hould be made starling from minimum resistance to a value which gives correct ratio. Should difficulty with dot distortion be encountered, the rule to follow is : if first dots of a series are foreshortened, add capacity to Cl and readjust R 1 ; if the opposite cond ition prevails, increase capacity of C2 and readjust Ri. Note that this rule applies only when distortion is present at a setting o f R1 where th.e correct dotdash ratio otherwise prevails. Operating Controls

Transmitter high voltage off or on is comrollcd by a switch (SW2) in the transform.e r center tap. This swit:Ch is ne..'TAANSt.11TTtA.

'J.

SWI

KtATtAS

~2- 10 I, and many of the required circuit points are also terminated there; so me of the wiring "will be accompl ished by installing jumpers on the mating connector, J-40 I. I. Install wiring to control the guard channel receiver by running a lead ·from pin I of term inal board E-10 I to one side of the new "Net Channel Receiver" toggle switch. G round the other side of this switch. Complete the circuit by installing a jumper from pin 6 to pin 18 of connector J-40 I. 2. Connect B+ to the voltage regulator droppi ng resistor and one contact of the v/o spotting switch by running a lead between these points and hence to pin 2 of terminal board E-10 I. Complete this circuit by connecting a jumper between pin 15 and pin 19 o[ connector J-401. Co nnect the si ngle insulated lead in the vicinity of R-176 to the junction point of R- 176, R-1 77, R-178 and R-18 1. Connect the other end of th is lead to the vacant v/o spotting switch terminal. 3. Connect 28 volts oc to the voltage regulator sub-assembly and the panel mounted pilot light by connecting the lead from pin 2 of the ballast tube to pin 14 of terminal board E-101,

34

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Ll-14 turns #22 wire on :Y." slug tuned ceramic Corm. L2- Prhnliry: 10 turns #2G unr1mel wire, clos~ wound over ground end o! the primaty. L2-Sttondary: 40 tum$ -#26 enam.el v.--irt, close wound on %" alui; tuned form. Cl, C2, C3-Silver MJca

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nal or the microphone tr.in.stormer and in it.s plnC'o connect the 3 volts or two Hnsblig.hl ooll.:1, grounding tlic ncitath •c. D i11connctL 111ic:l'uphone when n ol in use. c) ·rtrn t.rn111m1itter s hould n ow b e set

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Rear view of SCR-522 complete with automatic frequency mechanism mounled on top of lronsmiller.

52

The photo shows, above the receiver, n 7" x 19" panel with n 1-pole 4-posit ion switch, centered and to lhe leit-hand side. This switch controls the channel cont rol motor that selects any one of the four crystal frequency channels of your choice. Centerlo At.tnched io tho loo~• r.omponentoasiblc ao thnr.. tbe mensur~monta between tbo panel and the holes and mnrking for the anfe.le& l\ro accurate. The • hort end of tho angle is ouo drilled nt this time. The &ogles are placed into their p0eitiono with the ahorl end of the ••t" up,..,.rd. The panel is again corTeCtly placed against tho angles. n ac:ribtr is used to mruk the position of the holes on the :panel. A hole accord.ini:: to the siz.e of the fl&t screw used is drilled nL t.l1is mark through t he pnneL It is then countersunk ao tbnl tho bend is at panel love!. I) Wilh tho lraooinlltor and reeeivcr "" thoy are. po,.·er cable connections for U10 receiver are to be mo.de. Jones socket.

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Fig. 3-Circuit of the speech a mplifier and modulolor.

A schematic diagram of th e speech amp lifier used is shown but no construc tional details are given. If good construction prac tice is followed in building it, no difficulty should be encountered. A tune-operate switch is provided so that carrier might be inserted to tune lhe transmitter up. The "Tune" position of the tune-operate switch will also provide amplitude modulation.

control to m inimum and app ly voltage to the transmitter. If the scope has good sensitivity, a sm all amount of carrier wi ll appear as the balance control is rotated to either extrem e. T he balance control should be set for minimum carrier amplitude on the scope. This completes the initial tune up. The transmitter should now be loaded into an an tenna and the scope pattern watched as you speak into the microphone. Only the higher voice peaks should tend to be clipped or limited in the final. If heavy clipping is present, the drive to the final should be reduced by detuning the slug-tuned coil between the modulator and fi nal ( this m ight bother some of the purists, but it works) . Heavy loading should be used o n the final. If low-plate vo ltages (300-500 vol ts) are used, you might have to add another link to the final coil. This can consist of two or three turns of wire wrapped tightly around the bottom of the plate tank coil. If oscillations are present in the final, it might be necessary to add another .02 mmf by-pass condenser on the bottom of the plate tan k. This condenser may be soldered in parallel with the remounted condenser that was originally in the transmitter. Additional screen by-passing may also be necessary. T he parasitic a nd oscillation problem seems to va ry from tra nsmi tter to tra nsmi tter. Yo u are ' now ready to get o n the air and enjoy the advantages of sideba nd. True, mos t stat ions wi ll receive you as a single sideband station and this gives you a 3 dh disad vantage compared to a single sideband station of equal powe r. But, at least, you have gotten rid of the chief heterodyne cause, the carrier. Most of the s ideba nd boys will not know you are transm itting both sidebands unless you tell them. Sta tions as far as 1200 miles away were worked wi th the con verted transmitters running a peak power of only 35 watts. So if you have been looking for a cheap way to inves tigate this sideband business, here it is.

Power Supplies A word should be said here about power supplies. K9DBO's axiom in March 1959 CQ applies here also (quote ) "Good signals req uire good power supplies." The voltage applied to the oscillator should be well regulated by the use of series YR tubes. For best results, separate regulator tubes should be used for the 1625 screens and the oscillator; although a common regulated source did provide a satisfactory operation. The plate voltage may be anythi°ng from 300 to 750 volts. The ARR L H andbook may be consulted for the correct bias voltage for linear operation of the J 625's. Tune Up Initial tune-up procedure is not too difficult bu t docs require the use of a scope and preferab ly an audio oscillator. The scope should be connected to provide a two-to ne test pattern. The transmitter should be loaded into a dummy load in the tune position and the added slugtuned coil adjusted for maximum power output or cathode current. T he tune-operate switch should be thrown to the operate position and a 400-1000 cycle audio note applied lo the sp')cch ampl ifier. T he balance potentiometer should be adjusted until allernate lobes o n the two-tone test pauern have equal amplitude. If an aud io oscillator is not available, an alternate tune-up procedure is to couple the scope pickup loop tightly to the final tank. Turn the aud io

63

THE ARC-5 AS A LINEAR condensers, add a closed circuit jack for metering. T he con nection from R-74 and C-58C to the ccntcrtap of T -53 secondary is also removed. Add an extension to C-67 for final tuning. A 0 to 1ma meter in th e 1625 grid return will show a slight rise on peaks and is actual ly a worth-while operating aid in AB-2 operation. AF0HAJ has commented on AF5LHX's conversion (above) by adding th at 150 watts P EP is ava ilable but with 400 volts on the sc reens and I000 volts on the plates 250 watts PEP is available (th is grea tly exceeds the tube ratings ... Ed.) He tunes u p by using only 150 volts on the screens and then switc hes to 400. If additional link inductance is necessary wind two or three lurns of well insulaled wire at the bottom of T-54 in the same direction and connect in se ries with the link.

We got a fine publication from the Central Kansas Radio Club which has a conversion of the ARC-5 transmitter to a single band linear amp lifier. While we haven' t tried it out it should work well and add some more power to any station at little cost. Essentially they use the I 625's as before but change the 1629 and crystal to VR-150's, by rewiring, and thereby hold the 1625 screens to 300 volts. The lower terminal of T-53 (oscillator transformer) is removed from C-62 and brought out for bias. Bias is supplied by a pre-amp power transfor mer a selenium rec tifier, a 40 mfd J50 volt capacitor and a 30K potentiometer for adjustment. The 1626 oscillator is removed from its socket and R-73 cut out. A coaxial line, with the shield grounded, is connected to T -53 for signal input. Bypass the 1625 cathodes and filament with 0.0 I mfd disc ,.---- -

R72

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64

Fig. I. An attractive appearance is easy to attain with 274 -N transmitters.

THE SCR-274N SERIES HE SCR-274 N CO:\!MANO SETS have unqucstion· ably been the most pormlar of all war surplus equipment~ for co11 versio11 to amateur use. T he transmitters ar c by far the most useful or a ll units included. They not only arc used as transmitters di rect ly, llllt arc also very popular as exciters and vari able frequency osci llators for la rge r transmitters. As is the case in most wa r suri1lus items. the amateur use of these excellent little transmitters retruires a certain amoun t of modification. Ar ticles too numerous to mention ha,•e been written describ· ing various ways 0£ modi£ying the units for amateur service, each modification having its own particular advantages and disadvantages. With such a well· cjcsigned piece of equipment to start with, it is reasonable to expect that the most satisfactory modi· ficatiun would be the one that disturbed the original circuitry the least. Actua lly, very little is required in the way of modifications to the sets which cover t he ama teur bands as is. For i11 stancc, the I3C-690-A and the I3C-459-A cover the 3.4-1 me band and the 7-mc band respectively. The only absolutely necessary modification to these two units is the 11rovision o f a suitable output connection and a means for operati·ni: the keying relay. It is also generally de· sirable to parallel the heaters for 12- volt operation. If the unit is to be used as a VFO for driving a

T

65

string of multipliers £or operation in the higher amateur bands, it is wise to provide for. operating the oscillator heater from rectified and fairly well filtered power. Th is is to 11revent the slight amount of frequency 111odul:1tion :it :i GO·cycle rate which is present in some instances when raw a.c. is used £or t he oscill;,tor tube heater power. This frequency modulation is not suffidcnt to be noticeable on the fundamental or C\'Cn at twice the oscillator frequency. It is also not present in all sets. It is present in some, however, and in some instance, is very noticeable on 28 me. T he power requi rement for the oscillator tube is rather low. and a 250 ma selenium rectifier followed by about 250 ftf of capacity (dry electrolytic) is all that is required. Listc11 to the 28-mc harmonic or the transmitter with a good receiver. If no a.c. Imm is noted, it is safe to say that your particular unit is free from serious 60-cyclc £requency modulation. The original schematic diagram is shown in Fig. 2. The modification preferred by the :rnthor appears in the modified schematic diagram, Fig. 4, which includes the changes required to produce satisfa(tory keying to Ile 1kscribed. Numerous ways ha,·e bt'en suggc~ted for changing the fn-;111cnry llf t he BC-457-A (4-5 ..l me) and the BC-4.58-A (5.J-7 me) to the amateur frequencies.

L52

ANTENNA TUMNG

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Fig. Z. The original circuit of the Z74:N series transmitters.

Most of the suggested systems involve modifications to the inductances of the oscillator and P. A. tank circuits. The BC-457-A may be changed to cover the 3.5 me amateur band by the simple addition of a good quality capacitor of between 25 µµf and SO µµf capacity in parallel with the oscillator tuning capacitor, and by a readjustment of the variable padders in both the oscillator and P. A . plate tank circuits. The BC-458-A may be adjusted to cover the 7-mc band by simply adjusting the oscillator and P. A. padder capacitors. To adjust the BC-457-A to frequency after adding the capacitor to the oscillator circuit, insert a 4-mc crystal in the crystal socket. (An FT-243 type cryst.il is excellent for this purpose, and is inserted in the socket using pins 3 and 7.) Ne.xt, remove the cover from the oscillator coil and capacitor a' scmbly, using care not to disturb the iron core setting

(screwdriver slotted screw on top, sealed with blue glyptaJ). Cut a screwdriver opening in the end of the shield opposite the variable capacitor shaft. Loosen the setscrews which lock the capacitor shaft, and replace the shield cover over the oscillator tuning assembly. Turn on the transmitter, and with the tuning eye in place, adjust the main tuni1ig dial to 5.2. Adjust the oscillator padder until the eye indicates resonance with the crystal. The oscillator is now operating at 4 me. Remove the oscillator tuning assembly cover, tighten the capacitor shaft lock setscrews, and replace the cover. Tune the fuced padder of the P. A. stage for resonance as indicated by a minimum of P. A. plate current. If the iron cores in the tuning coils have not been disturbed, or if the coils have not been modified, the tuning will track over the entire range, and the new range will be 3.4 to 4.1 me. The dial may be

Fig. 3, 1Ru1trating the locat ion of the mounting holet for the time const ant capacitor C69.

66

C~A

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L52 r7•ing circuits have been tried with this v.f.o. For t he author the most satisfactorv has been to use relay KSJ and key the cathodes the 162.S amplifiers. This relay will operate nicely from a 22-volt battery, or through a 10-watt 17,000· ohm resistor from the 300-volt supply. The relay draws about 16 ma from ei ther the battery or the d ropi>ing resistor.

of

73

THE BC-45SA & BC-459A ON 15 Command-Set transmitters can be converted to cover the 21-mc band with a minimum of effort. Two conversions, applicable to the BC-459A (7-to9.1 me.), and the BC-458A (5.3-to-7 me.) transmitters are described below. The simpler conversion consists of shifting the oscillator tuning range to cover 10.5 me. to 10.725 me. and operating the parallel 162S's as frequency doublers. To accomplish these objectives, do the following:

Next, remove three turns from the top of the amplifier tank coil. The amplifier padder now becomes the main tuning condenser. Remove its shaft lock, and add a shaft extension, plus a knob, for easy adjustment. It may be necessary to dismount the padder from the chassis to remove the shaft lock and to enlarge the hole in the side of the chassis to accommodate· the shaft extension.

+HV

12V

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NQTE UNLISTE!> PARTS ORIGINAL AND UNCHANGED. LI- 7T•22 PE WIRE LONG, •DIA CERAMIC SLUG·TlJNfll FORM. L2- ~ T, DOUBLE SPACED ON ORIGINAL AMPLIFIER COIL FORM.

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Fi9. I. Modification schematic. The dotted llne1 enc:IOff the optional doubler stoge, which affords odded output a11d dablllty. BC-459A

Oscillator: Decrease the capacity of the paddini condenser, locat.e d under the oscillator shield can; so that the oscillator tunes to 10.725 me. when the dial is set to 9.1 me. The shield must be in place while checking frequency. AmflliMr: First, discoamt'Ct both the amplifier tuning condenser (the one ganged with the oscillator condenser) and the neutralizing condcruer.

Mount a midget phone jack in the lower lcfthand comer of the 1>11nel, and connect it between the 1625 cathode.$ and ground. Bypass- the cathodes to ground with a .O()l ,.,.£. ceramic condenser. Connect the power supply, plug a key in the jack, check your frequency, 'tun~ the 1625 plate -N.nk to reaonance, couple an antenna, and you are on 21 me. BC-458A

In addition to the

74

abo~,

putting the BC-458A on

21 me. requires modifyin&' the oscillator coil as well as the amplifier coil. Remove five turns from the top of the oscillator coil, and rewind the amplifier coil. T he new winding consists of five turns of the original wire, wound to occupy every other groo ve un the coil form. ( Usi11g a cot1v mtiv11al freqrmrcy doubler as a 11 out/mt stage always it1curs tire risk of radiati119 appreciable Put. The link should be coupled loosely to the coil to obtain good output. Experimentation with your pa1 ticular a pplication is the best condition under which to adjust the unit. Once the unit is adjusted correctly it will work with no trouble for a long period of time. The 2E26 tube plate should not get red under any condition if the unit is oper:lting properly. Short overloads arc permissable in tuning up , however. The 651\7 transformer may need a little adjustment when you are initially tuning for good output on tbc I I-meter band. By tuning it a little, a happy med ium can be struck that will be good for both 80 and I I.

Calibration To calibrate this unit so that it will be accurate in frequency readings, all t hat is necessary is a 100-c crystal oscillator and a receiver. II your particular installation does not require accurate calibration, you may calibrate it with the receiver alone if your receiver calibration is fairly accu rate. Since the original dial readings are discarded, a mask made from stiff paper wi1s cut out to fit over the dial. Tune the receiver to 3.5 me and tune the VFO to this spot. Make a mark on the new VFO dial at this spot. Now put the receiver on ten meters and find th e harmonic on 28.0 me. Keep the VFO tuned to 80 or 40 meters so as to get a weak harmonic to beat with the weak 100-kc harmonic at that frequency. Adjust the VFO trimmer to set the 3.5-mc point exactly at 28 me, for any bit you were off on the adjustment on 3.5 me will be multiplied 8 times on this band. A ftcr this is set, make a mark a t every 100-kc point from 27 to 30 me. This procedure will make your unit a pretty good frequency meter, but remember, when using it near band edges, join it with a 100-kc crystal oscillator to make an unbeatable team. In time, the VFO will t end to get off calibration. An adjustment of t he original oscillator trimmer will put it right on the beam. Results I have b~n using this exciter to d rive pp 807s on all bands for about a year now and have found nothing lacking in its capability of T9x c.w. I have never gotten a report be.l ow T9. Because I can now use breakin on the higher bands and st ill retain a good nutc, my scores in the DX and CD contests h;ivc been vastly improved. I !inti that this exciter retains its calibration over long pcriotls of time a nd is c1ui1c accurate as to band edges. Although I spent quite some time tak ing the bugs out of thi s unit, it is repaid a millionfold when I can say during a QSO, "Exciter hr OM is a lrome.,,,adc bandswitching job and I am using Bk in," after receiving a T9x report.

103

AN SCR-274N VFO FOR AM,FM of surplus Army

appearing on the civilian marAequipment ket. Among various items. of interest to the N

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il68

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Jt-' ..ot-- ~ = GND ~ :} +200V. ( Rt9uloted) 56ood t • (24V.A.C. t Amp.I 7- AnytNn9 up ti>+550V, O.C. occordin9 to power output.

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Fig. 2. Modified dia gram of transm itter covering from 5.0 to 7.3 megacycles. Referring to Fig. 1., the following components were e liminated mainly from the el ectronic 'e ye circuit: R.,, R.,, R, 0, R,., R,,, Y11, K.,, K,. and L.,. Parts added for the FM reactance modulator compriseC ,-.00025 µf mica R,-500,0 00 o hms gain control C,, C,- .01 µf piper RFC- 2.5 mh r.f chokes C,, -.0005 µI mica T,-line input audio transformer, 500 ohms to grid R,- 1,000 ohms, 1/2, watt carbon 12SJ7 metal or glass tube R,, R,-50,000 ohms, 1/1, watt carbon

eration frequency. This would enable the opera.tor to place himself exactly on a particular frequency in the band.

A dditional Modifications for FM I t is probable that the amateur will engage extensively in FM narrow-band operation in the near future as well as amplit ude modulation. Advantages are claimed for FM in services

closely paralleli11g amateur operation, such :l.:l mobile police and point-to-point communications. Not the least among these features is the very modest requirement in regard to modulating power. Also, existing supcrhets will do a good job of receiving FM transmissions. Later, of course, an FM channel wiU no doubt be standard equipment in all luim receivers. By making a few more addition:i.l ch:i.uges, t he

UNIT BUILT FOR "PLUG IN" INTO CRYSTAL SOCKET SHIELDED TWISTED PAIR ( UP TO 50 FT.)

( Coox. Jin• is also sotlsfoclory) FM-AM VFO UNIT

~ I

t LC - RESONANT TO SAME FREQUENCY OF VFO FM - AM UNIT. Ct - GRID BLOCKING CONDENSER, ABOUT .OOOSµf

f ig. 3. Suggested mangement for coupling the VFO to the regular station transmitter when high power is desired

107

This shows th11: bottom view and is practically as is wh11:n it comes From the .. ARMY. The reactance tube and components are mounted in the rear. The gang ing of the Mast11:r oscillator and power ampl ifi11:r cond11:nsers is clearly shown. Notice worm g11:ars on the condensers.

already modified AM VFO can just as easily be converted to narrow-band FM operation. Thls is accomplished by adding a reactance modulator bube and shunting its output circuit direet.ly across the master oscillator tube-thereby varying the frequency of the master oscillator in accordance with the audio applied to the reactance ; ube input circuit. Of course purely AM operation is still possible as above. The FM feature is additional. Again referring to the original and modified schematics, the electronic eye (1629) is removed to make way for the substitute reactance modulator tube. This new addition will be a 12SJ7 type tube. Also the resonator crystal is dispensed with, and all wiring from these two sockets removed, ·with the exception of the beater leads to t he 12SJ7 tube. Note that the original resistor R71 remains in t he circuit across the .heater terminals. The new wiring is simple and follows that in the modified schematic. A 500-ohm l.ine to the grid transformer permits the output of the speech amplifier to modulate tbe reactance tube. Audio required is approximately zero db. Should Al\1 operation be desited, it is merely necessary to turn off t he reactance tube gain control, R4, and p late modulate the station t ransmitter in t he usual way. With FM operation, the zero db audio track is fed into the 500-ohm in put circuit and the gain control turned up sufficiently to produce t.he required swing of the carrier. Of course the mean frequency may be spotted anywhere in the band by using the calibrated dial in t be usual way. Needless to say, it is necessary, when using FM, to stay within the confines of the FM portion of the band. A swing of a few kilocycles on the fundamental frequency of tbe VFO will be multiplied by the same ratio of frequency multiplication in the transmitter. Thus, if FM operation in the 144-148-mc band

is desired with a VFO frequency of 6 megacycles, a swing of 1 kc at tbis point will be multiplied by 24, which is more t han ample for narrow-band amateur FM work. Coupling to Main Rig The out put of the FM-Al\il VFO unit can be

coupled to the crystal tube of the regular station transmitter in a number of ways. At W5AJG, a shielded twisted pair runs from the operating desk, upon which the VFO is mow1tecl, to the crystal stage of tbe transmitter proper across the room (Fig. 3). The crystal is removed and a separate t uned tank circuit substituted by plugging into the crystal holder. Should the excryst.al t ube be a harmonic type, this tuned tank can be of the same frequency as the crystal. In tri-tet crystal oscillators, the cathode coil should be shortened. Wit h pentode t ype oscillator tubes, it is usually possible to work straight through without self-oscillation . However, should 7-mc operation be primarily desired, it is advisable t-0 choose a VFO Uiijt operating on 3.5 megacycles so that the former crystal-controlled tube will opemte as a doubler. In any event, care should be taken to avoid shorting the grid bins of the excrystal tube by connecting a blocking capacitor in series with the high side of the oscillator tube. It will be found t hat t he SCR-274-N makes a very nice VFO unit with AM or FM operation optional at a very low cost. It is suggested that those interested in obtaining equipment of tbis type, contact firms that rebuild and reconvert government aircraft apparatus to civilian requirements. As used in Army service there is usually about three times the amount of equipment needed for civilian purposes, and the surplus gear is generally available at a very moderate cost.

108

A SURPLUS SSB EXCITER Anyone who has triod to build a variable-frequency exciter that produces a T9 signal, keys well, and is stable enough to drive an SSB transmitter appreciates the difficulties involved. W6TZB, however, shows how to let someone else do most of the sweating.-Editor

While working on a sing le-sideband rig recently, the writer developed a simple method of con verting 3 to 6-Mc and 6 to 9-Mc "Command" receivers (BC-454 and BC-455) into heterodyne-type, variable-frequency exciters, covering the same frequency ranges. The stabi lity of the converted units is outstanding, they key beautifully, and have sufficient output to drive a Class AB 807, a 6AG7, etc. Heterodyne exciters have been described in the radio magazines for quite a few years; Their manifold virtues include good keying, excell ent stabili ty, and high reset accuracy. H owever, they a re rather complicated and expensive to build. In addition, many arc difficult to adjust and their output is full of "birdies." The conversion of a "Command" receiver into one, however, retains the advantages and eliminates the disadvantages. Besides the Sl4r{Jl11s receiver, a smal1 handful of fixed condensers and resistors and an Ol)tional crystal are the only parts required. Unless you are unfortunate enough to possess nothing but

Ct

What Must Be Done

To accompl ish our purpose, we change the receiver wiring around, so that, instead of highfrcquency input to the antenna terminal being converted to intermediate-frequency output from the mixer plate, intermediate-freciuency input to the mixer g rid is converted to high-frequency output at the a ntenna terminal. Figw·c 1 shows essentials of the high-frequency section of the receiver , where most o{ the simple, though necessary, changes are made. Fig 11re 2 shows the revised c.ircuit. ·Starting at the antenna term inal, change the present coupling condenser to 100 µµfd. and remove the small neon bulb. Disconnect C3 and Rl from the control g rid (pin No. 4) of the 12SK7. Remove and save Rl. Now, transfer the plate connection ({Jiii No. 8) to the grid, and connect C3 to the plate terminal. Feed· plate voltage lo the terminal th rough a 2.5 mh. r- f choke and a 200-ohm resistor in series, bypassing the resistor to ground through a 0.002-µµfd mica or ceramic condenser. Disconnect the bottom of L2 from the 8 plus line, and ground. Believe me, this is important. P roceed ing to the 12K8 mixer stage, disconnect L.~ from the !2K8 g rid cap. Also disconnect the wire between the 12K8 plate (/1i11 No. 3) and the first i-f transformer. Then connect the plate term-

r ------- ---1

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thumbs, the entire job should take only an ·hour or two.

C2

I Fil/SY IF 7118£

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I

C13

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Fig. I. Skeleton diagram of the r-f a mplifier and mixer stages in either a BC·4S4 or BC-455, before modification to a variable frequency exciter.

109

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MIXER

12SK7

12K8

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Fig. 3. Thil is how the first i-f sta9e b converted lo o crystal controlled oscillator.

This adjustment' pr oduces maximum output at the desired frequency, commensurate with minimum spurious outputs. After the low· freq uency output is adjusted, adjust the paddus across L3 and the ttd to Rtod 0·300Mo oil olhtt

Poslllo••· 4A' - final Amp. Pia It Ma.

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'" Fi9. 3. Modulator circuit dia9ram. Frequency Multipliers

The frequency multiplying strip is a small subassembly built up on a piece of aluminum large enough to cover the rectangular cut-out in the chassis where the 162S's used to be mounted. The parts arc so laid out that the first 2EJO frequency multiplier grid is close to the tap on the secondary of the v.f.o. grid coil assembly. See Fig. 2 for v.f.o. coil connections. The plate coils and tuning condensers of the 2EJO's arc located near the edges of the aluminum strip with the two 2EJO's in the middle, allowing room to pile up the necessary plate, filament, grid and screen grid by-pass condensers. This arrangement allows the operator to tune the condensers and coil slugs through the access door on top of the dust cover that was formerly used to get at the 1625's. As shown in the photos, the 5516's arc mounted on the left side of the final tank condenser, and the coil is mounted on the right side. The antenna coupling link is adjusted by hand by bending its pigtails and the loading is adjusted by varying the series antenna condenser which mounts on the front panel. The link line bet ween the last doubler plate coil and the final grid coil is a short length of receiving type 72 ohm twinlead, anchored at each end on tic point.s. Coils

The 2E30 coils arc wound on National XR-50 slug-tunerl coil forms or equivalent. according to the coil table, and arc mounted and preadjusted to resonance with a grid dip oscillator before mounting the aluminum strip in the main chassis. The

grid coil of the final is wound on a plain ~· diameter coil form, and after adjustment with the grid dipper is cemented with coil dope. The final plate coil is wound with #12 wire and soldered to the condenser terminal!. To cover from 28.5 me to 29.7 me, the v.f.o. frequency range will be 7.125 me to 7.425 if a BC 459-A is used, requiring two doubler stages. If you arc using a BC457-A, the frequency range will be 4.750 me ti> 4.950 me, necessitating a triplcr and a doubler. If you are using a BC 696-A, the frequency range is 3.166 me to 3.300 me, following with two triplers. Of course, a BC 458-A can be made to tune the 7.125 to 7.425 me range by opening out the air padde.r that is in the shield next to the v.f.o coil, or to tune the 4.750 to 4.950 me range by closing in the same air padder. The latter will give better band spread, and that is what the writer did. Rotor plates may be removed from the oscillator tuning condenser with a pair of pliers for increased band spread. The writer ended up by removing 2/3 of the rotor plates .. This gave considerably more band spread on the V.F.O. dial than is shown in the photos. Care must be exercised in twisting these plates and pulling them out with a pair of longno~ed pliers. The force should be exerted with a twisting motion by the pliers between the plates and the rotor shaft and not with a straight pull between the plates and the chassis, as there is dan~er of pulling the rear rotor shaft bearing out of its socket (1£ this docs happen, be sure to catch all the tiny ball bearings so that the condenser may be repaired. This is done by removing the condenser from the chassis and removing the

114

rotor so that the ball bearings may be replaced. To do this, drive out one of the taper pins in the flexible shaft, remove the screws holding the condenser to the chassis, unsolder the connections to the coil and tube, remove the spring-loaded gears on the condenser shaft, and unscrew the bearing on the opposite end of the condenser shaft. The rotor now lifts out easily. Holding the condenser vertically with the shaft end down, drop the ball bearings into the race with a pair of twce.z crs and replace the rotor shaft. Holding the rotor shaft so that the ball s can.not fall out, revers~ the position of the condenser and replace the balls in the other bearing and then replace the screws. The condenser is now as good as new, and may be put back in the unit. If any balls arc lost, they may be replaced from one of the two condensers that you have previously removed. You may even practice on one of these before trying to remove plates from the V.F.O. condenser if in doubt.)

Modulator The speech amplifier-modulator unit is built on a SCR-274 transmitter chassis so that it may be plugged into a double shock-mounted transmitter rack alongside the transmitter. T he circuit diagram is shown in Fig. 3, and consists of a 2E30 triode

driving a 5516 clamp tube modulator. If desired, the reader can build up almost any type of modulator ; however, for the power ·involved and the overall battery drain, we decided in favor of the clamp tubc3. especially since no modulation transformer was needed. Since it is not feas'ible to use a r:ithnrle resistor with a lilnment type tube. a "C" battery is necessary to set the operating bias for the clamp tube. This same 45V battery supplies 'ixed bias for the Rf.' units and in this way provides protection for the 5516 tubes in the case of ~citation failure. A one mcirohm pot is con nected 1crnss the battery tor is screwed onto the re· ceiver fitting. The pkistO• _r-00"='"

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fig. 1- Powe r supply for the BC-348.

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Fig. 3-Schematic of the BC-348,

177

2 3 4

5 6 7 8

Table I Output Relay (short to 6 for receive) Plus 24V DC Plus 24V DC Output (may be grou nded) Relay (short to 2 for receive) Ground-24V D C Ground-24V DC

Several "S" meter circuits were tried, but with the lack of a calibrated meter and the effort needed in properly calibrating such a meter, we decided agai nst putting one in. Actually, unless you have such a meter, it would prove only to be a rather expensive tuning meter, and probably useless. If you feel that it is worth it, you can refer to the handbooks that are available for suitable circuits. The original BC-348 that we had used a set of binding post type of terminals for the antenna input. We found that the use of a coa.xial connector was more compatible with our equipment and we removed the one marked "Antenna" and enlarged the hole to 5,1i inch for a standard coaxial connector. The results were surprising, especially the reduction in noise pick up from the fluorescent desk lamp. Alignment

Now comes the alignment job. Set the receiver to mvc reception, with the bfo off. Beg, borrow or even buy a signal generator and connect the hot lead to pin l of VT-116 ( 6SJ7) Cn, and wire a volume control into the grid of the first audio tube. (i.e., the coil farthest from the base) . The lower coil should have 195 turns removed. L4, LS-Remove 147 turns from the top coil and remove 38 turns from the bottom coil. lst i.f.-Rcmove 800 turns from both coils. 2nd i.£.-Use the existing tap for the outside end of the coil, making sure to reconnect the tap to the enct of the coil. No tap connection was used. 3rd i.f.-Same as the 2nd i.f.

BANDSPREADING THE 274'5

A bandspread of 42 kc per division instead of the normal 100 kc may be obtained (on the 3-6 me receiver) by remov ing five of t he eigh t rotor pla tes on the tuning gang. Additional padd ing condensers must be added across the r-f and oscillator portions of the circuit. 1 fou nd 33-µµf NPO Ccramicons did the job. Another 274 trick is to lift the opposite end of the 620-ohm, resistor co nncs;tcd to pin 5 of the r-f am· pfifier tube aod ground this resisto r. thus rcmovingt he r-f amplifi er from the gain control line. This will improve the s ignal- to-noise ratio. The diagram shows the modifica tion required to obta in t his increased performan.ce.

181

12SK7

~---

Fig. 1. Showing the tuning dial.

274N DUAL CONVERSION With good 11rospccls ahead for the higher frequency bands, and as more hams get on 10 and 15, we hear the boys speaking, not, as the Walrus said, about cabbages and kings, but about beams and receivers. For many years past, amateurs generally have depended on the manufacturers to furnish them with receivers instead of making their own, and it must be said that the various companies have done a pretty good job. Performance on the low frequencies has been good, even on the low priced models. Above 14 me however, most of these receivers are lacking in sensitivity and image rejection, so that it has been necessary to use converters or preselectors ahead of t hem. In the high-priced newer model receivers double conversion is used, which practically eliminates images and at the same time increases the sensitivity on the higher frequency bands. However, when the Walrus mentioned cabbage, maybe he was thinking about the price of these new double conversion jobs, and the kings who could afford to buy them. Many of us simply cannot do so, and balance the budget. We do have our old modd receivers, hot upstairs, but not so hot downstairs, and they have continuous coverage from 550 kc to 30 me or thereabouts. If you are in that class, brother, read on: The 274N Receiver

Most hams arc familiar with the little "Command" rcceivel'3, the 274N Series, which are so plentiful on the surplus market. One of them, and only one, the BC 455, has an intermediate

frequency of 2830 kc. With a small power supply, capable of giving 24 volts a.c. for the heaters, and 250 volts d.c. or thereabouts for the plates, they may be used without modification on 40 meters, and they make good stand-by or portable r igs. By rewinding the r-f section they may be used as high up as 28 me with a surprising amount of sensitivity if the job of rewinding is properly done and the rig is realigned to peak the various stage.s. Used as a converter, with the 2830 kc output taken directly to a communications type recei ver, images are eliminated in the first conversion, while the old receiver takes care of the jobs of amplification, noise limiting, beat frequency production for c-w operation, and a udio control. Tuning Tuning is done with the BC 455. Band spread and logging dials may be added, concentric ·with the tuning shaft as shown in the photograph fig. 1. The method used here may be explained as follows: To the threaded sleeve which holds the splined tuning shaft in place is fastened a dial scale from a BC 375 tuning unit. This scale has a hole that just fits the knurled outer diameter of the threaded sleeve. The sleeve is first drawn up tight, the original tuning knob removed, and the BC 375 dial forced onto the sleeve with its number 50 pointing directly upward. A skirted knob is then installed on the tu ning shaft. The skirt of this knob just comes out to the bottom of the dial scale. Indicating arrows, numbered 1 to 4, are placed 90 degrees apart on the outer edge

182

of the skirt. By making use of the main dial markings, together with the numbered arrows and the auxiliary dial, stations may be accurately logged at any place on the tuning system. The BC 455 Command receiver was designed 10 cover a range of from 6 to 9 me. It may be

Fig. 2. Undorchassis vi&w tho r·F coils to tho left.

converted to cover hi gher frequencies , at least up to and including all of the "ten meter" amateur band, by simply rewinding the r-f coils with fewer turns. No circuit modification is necessary, nor is it necessa ry to remove condenser plates. Fig 2 shows one of these receivers with the bottom plate removed. The oblong strip near the left end holds the r-f coil assembly which consists of the three coils mounted on their respective sockets, and three square metal shield cans, as shown in fig 3. A complete assembly is shown at the left of the receiver in fig 2. By removing two small screws the assembly may be lifted off the small banana plugs which ma ke the connections to the coils. Properly assembled it cann ot be plugged in wrong. H owever. the loca tion and position of the separate coils in the shield cans is verv important. To help keep track of these things small dabs of brightly colored paint, known as "witness marks" are located in their proper positions on the shields and coil sockets. I spoke of three coils. Actually, there are five coils, wound on three forms. Two of the coils have resistors attached to one end, and running to another plug on the coil socket. These resistors are left as they arc. The coils are designated LI, L2, L3. L4 nnd L5, and arc shown with the ir rcspec1ive terminals in the sketch fig 4. The coil forms arc fastened to their metal socket bases, and they are best left there while they nrc being rewound. This way, each

Fig. 3. Tho r-F coils removed From their shield can

~ .

,- . •

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.

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=• l.

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'

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.

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coil may be plugged into position separately and tested with a grid dip meter if one is available. The coil shields seem to have little effect on the frequency, any difference may be compensated for by the trimmers and the slugs may also be used to peak the coils. Starting with LI, remove all but six turns, spacing these turns over about ha lf of the coil form so that the slug will have an effect on the inductance when moved in or out. The inductance of this coil should be such that the little antenna trimmer knob on the front of the BC 455 will be effective at the desired frequency. If it is not, it may be necessary to adjust the tum spacing a bit. L2 is the r-f mixer coil. It originally bas a "honey-comb" winding. Unwind all of it, but be sure to leave the wire long enough so that you will have ten turns to wind back on. These ten turns will later be interlaced in between the turns on coil L3 so as to provide very close coupling for greater sensitivity. L3, also a mixer coil, is rewound with six turns, which may laier have to be reduced to five if necessary. Space the turns the full length of the coil form, plug it into position in the receiver and test for resonance with a grid dip meter. The tuning dial should be set at the place where you want the band to come. Adjust turn spacing and slug position until the inductance is correct. Tuning the main dial should dip the meter. In this way you can locate where the band edges will come on the dial. L4. It is not necessary to modify this coil at all.

Fig . 4.

L5 should be very nearly a duplJcate of L3. Start with six turns, spaced fairly close, about ~ inch from L4. With some adjusting of the slug and turn spacing it can be made to match the frequency of L3 without much trouble. When re-assembling the coils for tria.1, use just enough of the little screws to serve the purpose, as it is very likely that you will have them out and in several times. This can be done quite rapidly once you get the hang of it. If a signal generator is available, or can be borrowed, it will help the trimming process. The i-f coil train in the BC -455 should first be aligned for best output. Jn fact, the receiver should be first tested for correct performance on 40 meters before any changes are made.

183

A small amount of trim.mer adjustment may be necessary to make the receiver work as it should on 40. The i-f transformers need to be aligned but once, but the r-f end will of course have to be re-aligned after re-winding the coils. Trim.mer condensers for this purpose will be found at the front of the receiver, top side. Some units have holes in the covering shield

vrm

12SR7

VT64 ~2A6

•

Pin No. 5 on the plug should be connected to the shield of the cable. This cable runs directly to the antenna terminals on the Communications Type Receiver with which the converter is to be used. Set the dial of the second receiver to 2830 kc, fire up both receivers, and you should be in business. You will need a volume control on tbe BC 455. The b.f.o. won't operate, as you have pulled the tube. Probably the volume control on the second receiver will have to be set pretty low. CW is received by throwing in the b.f.o. in the second receiver. You may find that the noise limiter in the second receiver works much better than it ever did. The "S" meter will read a lot higher. Crystal selectivity filters or "Q" Multipliers will work in a normal fashion. Images will be absent, and the signal strength, as compared to that of the old receiver alone, will be very gratifying.

Caution

Schematic of adaptor plug.

It might be well to add a word of caution here; Be sure that the input circuit to the old to allow for their adjustment. If they are not receiver is not open. These input coils are somethere, better make them. The first two at the times wound with a few turns of very fine wire, left, as you face the receiver dial, are in par- so small that a good heavy dose of static will allel. One is set either all in, half way in, or all open them up. It has occurred, in several cases. out, and the trimming done with the mate. Surprisingly enough, the receiver will work to a These tune the mixer coils. A little farther to certain extent even with the input coil open. the right there are two more, also in parallel. The foregoing winding data is given with the These tune the oscillator coil L5. At the ex- assumption that the BC 455 is to be used treme right there is a single trimmer. This one mostly for ten meters. lt is possible to squeeze trims the coil L4, and is best adjusted when. the 10, 11, and 15 meter bands into one set moved· in conjunction with the tuning dial for of coils but it will take very careful adjustbest overall receiver output. ment of turn spacing and trimming to get the proper tracking, because the ten meter segment will be at one extreme dial reading and the Operation fifteen meter portion will be at the other extreme end. It is much easier to convert two With everything trimmed up properly the re- units, one for use with 10 and 11, and the ceiver should work very well as a straight ten other one for use on 15 and 20 meters. In the meter receiver on fairly strong signals while latter case, start with double the number of using headphones. When this bas been ac- turns given for the various coils and trim down complished we can take up the job of making a until the band segments are at the desired converter out of it. This is easily done. All you places on the main dial. have to do is to remove the second detector tube, VT 133, 12SR7, and make up an adapter plug to put in its place using but two of the pins, No. 3 and No. 4. It is not necessary to remove the audio output tube 12A6. since its The receiver in the author's shack. heater is disconnected when the 12SR7 is removed. The adapter should be made long, for two reasons; A short adapter would be very difficult to plug into the socket. Also the adapter should contain a small condenser in the lead to pin No. 4. An old coil form, large enough to take a plug at one end, and about three inches long would be ideal. The condenser, .001 >tfd. could be inside the tubing, with one lead running to 9in No. 4 on the plug. The other condenser lead should go to the inner conductor of a piece of small co/ax cable.

184

THE R28 VHF RECEIVER This unit was dcsign~d by the Weste rn Electric Co. for operation on 24-28 volts. It fits into an FT 220·A mounti ng rack. The line· up is a 7 17A in the r-f stage, 717A as a mixer, 12SH7 oscillator and two more 717A's as multipliers, two i·f